The first European spacecraft to visit Venus has settled safely into orbit of Earth’s “sister planet”, after firing its main engines on Tuesday morning. The Venus Express mission aims to find out why the planet – often also dubbed Earth’s “evil twin” – ended up like a vision of hell.

The probe blasted off from Kazakhstan on 9 November, 2005. It arrived at Venus this week, having travelled 400 million kilometres in just five months.

Mission controllers at the European Space Agency told a press conference in Darmstadt, Germany, that the morning’s thruster firing all went according to plan. “I’m very proud of the way everybody’s performed on this mission,” said David Southwood, ESA science director. “It’s been spectacular.”

Initially, Venus Express will adopt an elongated orbit around Venus. Further engine burns will nudge it into its final orbit by 7 May 2006. This elliptical orbit will carry it over the planet’s poles, swooping down from a peak of 66,000 kilometres above the surface to just 250 kilometres above the ground at its closest point.

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Greenhouse hell

Scientists expect to see some preliminary images from the spacecraft this week. The probe will continue to explore the planet for at least two Venusian days – the equivalent of 486 Earth days.

Venus is roughly the same size as the Earth and probably started out with a similar chemical composition. But, unlike the Earth, Venus is ravaged by a runaway greenhouse effect and its average surface temperature is about 470°C – hot enough to melt lead.

The atmosphere on Venus is almost entirely carbon dioxide, and the surface pressure is nearly 100 times that of Earth, equivalent to an ocean depth of 1 kilometre. And rain clouds in the Venusian atmosphere spit out corrosive sulphuric acid.

Wind speeds at the cloud-tops are 50 times faster than its surface rotation rate, and there are giant vortices over the planet’s north and south poles that defy explanation.

Volcano watch

To shed light on these mysteries, instruments on Venus Express will make global maps of the composition of its atmosphere. They will also make the best measurements yet of the weather systems, mapping the temperature variations, cloud formations and wind speeds at different altitudes.

The spacecraft should also find out if there is active volcanism on the planet. Radar observations have revealed that Venus’s surface is pock-marked by thousands of volcanoes.

Venus Express will also look for disturbances in the atmosphere that might come from eruptions or quakes. “If there are clouds of sulphuric gases coming out of a volcano, we may be able to see that,” says mission scientist Colin Wilson from the University of Oxford, UK.

Overall, the mission should clarify the bigger picture of why Mars, Earth and Venus ended up so different, and why Earth seems to be the only planet capable of fostering life.